The present invention relates to an exhaust gas recirculation cooler, which is provided in an exhaust gas recirculation line departing from an exhaust system and opening into an intake system of an internal combustion engine, and is configured as a heat pipe heat exchanger including at least one enclosed and evacuated heat pipe filled with a working medium, the first end of said heat pipe being subject to exhaust gas from the exhaust gas recirculation line and the second end being in contact with a heat sink.
In order to obtain lower NOx emissions in internal combustion engines it is known in the art to recirculate part of the exhaust gas stream from the exhaust system into the intake system. The NOx emissions are reduced directly upon combustion, by reducing both flame temperature and combustion rate. These mechanisms will lower combustion gas temperature during the combustion process despite the fact that the temperature of the intake air/exhaust gas mixture is increased by recirculating the exhaust gas. For a further decrease in NOx emissions it has proved of advantage if the two components of the admitted mixture (fresh air and recirculated exhaust gas) are additionally cooled off as far as possible. An internal combustion engine of this type is disclosed in DE 43 19 380 A1, for example. The EGR cooler usually is configured as a simple air/air or air/water heat exchanger whose working medium will not change its physical state.
U.S. Pat. No. 3,962,869 A and U.S. Pat. No. 4,107,922 A describe an internal combustion engine where a heat exchanger based on the heat pipe principle is provided in the exhaust system for the transfer of heat from the exhaust manifold to an exhaust gas reactor.
Another internal combustion engine with a heat pipe heat exchanger is disclosed in RU 2 070 655 C1, wherein heat is withdrawn from the exhaust gas of the exhaust system and transferred to a carburetor of the intake system.
U.S. Pat. No. 3,662,542 A describes an exhaust gas heating unit where energy is removed from the hot stream of exhaust gas of an engine and used for heating the passenger compartment on the principle of heat exchange. This unit is not provided in an exhaust gas recirculation line but in the engine""s exhaust system.
A heat pipe concentric with the exhaust pipe of an internal combustion engine is described in U.S. Pat. No. 3,737,286 A, though it is not provided for exhaust cooling but for after-burning of the exhaust gases by the addition of fresh air.
Heat exchangers operating on the principle of heat pipes are provided with one or more enclosed and evacuated pipes. Each pipe forms a closed circuit. Heat transport is effected by circulation of a special working medium in the pipe. By continuous evaporation and subsequent condensation of the evaporable working medium, such as methanol, the heat admitted at one end of the pipe is carried off to a heat sink at the other end of the pipe due to condensation of the working medium.
An exhaust gas recirculation cooler configured as heat pipe heat exchanger with at least one enclosed and evacuated heat pipe filled with a working medium, whose one end is disposed in the exhaust gas recirculation line and whose other end is connected to a heat sink, has been published in AT-GM 3.888. The first end of each heat pipe in the exhaust gas recirculation line is subject to the recycled exhaust, the flow direction of the exhaust gas being essentially normal to the axis of each heat pipe (transverse flow). The second end connected to a heat sink is subject to the gaseous or liquid cooling medium. The heat sink may be formed by the engine cooling system or the heating system for the passenger compartment.
It is the object of the present invention to further improve an exhaust gas recirculation cooler of the above type by keeping the number of pipes small whilst maintaining full performance, such that a lighter, simpler design requiring less space will be obtained.
According to the invention this object is achieved by providing that the longitudinal axis of the exhaust gas recirculation cooler, to be called EGR cooler henceforth, be aligned in parallel with the axis of the at least one heat pipe, so that an essentially axial flow of exhaust gas will be obtained in the EGR cooler. The longitudinal flow will result in a very compact design, improving cooling performance and reducing pressure losses.
Particularly efficient cooling of the recycled exhaust will be obtained by using a pipe-shaped design for the housing of the EGR cooler and dividing it by a supporting plate, so that a first compartment subject to the exhaust gas and a second compartment subject to a cooling medium is obtained, the supporting plate holding a bundle of heat pipes whose first ends project into the compartment subject to the exhaust gas and whose second ends project into the compartment subject to the cooling medium.
According to the invention the heat pipes are disposed in at least one concentric circle around a central heat pipe, the compartment of the EGR cooler subject to the exhaust gas being provided with an essentially axial exhaust inlet opening on one end and an essentially radial exhaust outlet opening on the opposite end. In a preferred arrangement one central heat pipe is surrounded by six concentrically disposed heat pipes. In order to improve uniform flow along the individual heat pipes a diffuser may be provided which departs from the exhaust inlet opening and expands in width towards the heat pipes, the central heat pipe projecting beyond the other heat pipes and extending into the region of the diffuser.
It is provided in a particularly preferred variant of the invention that the compartment of the EGR cooler subject to the exhaust gas be provided with two concentically disposed pipes in the vicinity of the exhaust outlet opening, which form a toroidal space, the inner pipe adjacent to the diffuser forming a gap-shaped opening together with the supporting plate. The stream of exhaust gas thus passes along the individual heat pipes before entering the toroidal space in the region of the supporting plate, which space contains the exhaust outlet opening. In this way short-circuiting of the exhaust stream from inlet to outlet opening and an uneven flow past the individual heat pipes is prevented. It will be of special advantage in this context if the width of the gap-shaped opening is non-uniform, the smallest width of the gap being provided next to the exhaust outlet opening. On the side of the outlet opening the inner pipe may extend up to the supporting plate.
Depending on a potential temperature gradient in the EGR cooler the heat pipe diameters and/or depths to which the heat pipes are immersed into the recycled exhaust and/or heat sink may vary. Within one and the same EGR cooler the type of working medium and/or filling level may differ between at least two of the heat pipes.